NASA Technical Reports Server (NTRS) 20110006891: Mechanisms of Diurnal Precipitation over the United States Great Plains: A Cloud-Resolving Model Simulation

Mechanisms of diurnal precipitation over the United States Great
Plains:

A cloud-resolving model simulation

Lee, I. Choi, W.-K. Tao, S. D. Schubert, and l.-K. Kang

The mechanisms of summertime diurnal precipitation in the US
Great

Plains were examined with the two-dimensional (2D) Goddard
Cumulus

Ensemble (GCE) cloud-resolving model (CRM). The model was
constrained by

the observed large-scale background state and surface flux derived
from

the Department of Energy (DOE) Atmospheric Radiation
Measurement (ARM)

Program’s Intensive Observing Period (IOP) data at the Southern
Great

Plains (SGP). The model, when continuously-forced by realistic
surface

flux and large-scale advection, simulates reasonably well the
temporal

evolution of the observed rainfall episodes, particularly for the
strongly forced precipitation events. However, the model exhibits a
deficiency for the weakly forced events driven by diurnal
convection.

Additional tests were run with the GCE model in order to
discriminate

between the mechanisms that determine daytime and nighttime
convection.

In these tests, the model was constrained with the same repeating
diurnal variation in the large-scale advection and/or surface flux.

The results indicate that it is primarily the surface heat and
moisture

flux that is responsible for the development of deep convection in
the

afternoon, whereas the large-scale upward motion and associated
moisture

advection play an important role in preconditioning nocturnal
convection. In the nighttime, high clouds are continuously built up
through their interaction and feedback with long-wave radiation,
eventually initiating deep convection from the boundary layer.
Without

these upper-level destabilization processes, the model tends to
produce

only daytime convection in response to boundary layer heating.

This study suggests that the correct simulation of the diurnal
variation

in precipitation requires that the free-atmospheric destabilization
mechanisms resolved in the CRM simulation must be adequately
parameterized in current general circulation models (GCMs) many of
which

are overly sensitive to the parameterized boundary layer heating.